The present invention relates to a dynamic pressure-type liquid bearing unit.
Conventionally, a spindle motor using a dynamic pressure-type liquid bearing unit has been used in an information apparatus such as a magnetic disk drive unit, and, for example, in Japanese Unexamined Patent Publication No. H9-210054, a dynamic pressure-type liquid bearing unit which prevents lubricating oil from leaking is disclosed.
FIG. 5 shows a prior-art dynamic pressure-type liquid bearing unit.
A thrust plate 4 is attached to one end side of a sleeve 3 which is rotatably supported on a shaft 2 as a stationary shaft, and a thrust bearing 1 attached to the shaft 2 is sandwiched by the sleeve 3 and the thrust plate 4 with a gap therearound. A sealing plate 5 is fixed-by press-fitting on the other end of the sleeve 3, and a minute space is formed between the inner circumferential surface of the sealing plate 5 and the shaft 2. In addition, a motor hub 6 is adhered by press-fitting on the outer circumferential surface of the sleeve 3.
Lubricating oil is filled in the gap between the shaft 2 and the inner circumferential surface of the sleeve 3 and the gap around the thrust bearing 1 between the sleeve 3 and thrust plate 4 and, and in order to prevent leakage of the lubricating oil, first and second capillary sealing portions 7a and 7b are formed on both end sides of the bearing gap.
Namely, the V-shaped first capillary sealing portion 7a, on which a tapered surface along the axial direction is formed, is formed between the upper end portion of the inner circumferential surface of the sleeve 3 and the shaft 2, and the V-shaped second capillary sealing portion 7b, on which a tapered surface along the axial direction is formed, is formed between the thrust plate 4 and the shaft 2. When the first and second capillary sealing portions 7a and 7b are formed as such, the lubricating oil is retained in the bearing by capillarity, therefore leakage of the lubricating oil can be prevented.
In addition, at least on one surface of the outer circumferential surface of the shaft 2 and the inner circumferential surface of the sleeve 3 that is opposed to the outer circumferential surface, herringbone-shaped dynamic pressure-generating grooves 8a and 8b are formed in the section between the first capillary sealing portion 7a and second capillary sealing portion 7b. Herein, such surface that is formed with the grooves 8a and 8b is the inner circumferential surface of the sleeve. In FIG. 5, the grooves are illustrated with the same height as that of the sleeve 3 and in a laterally shifted manner.
The shape of the dynamic pressure-generating grooves 8a on a radial bearing surface formed on the inner circumferential surface of the sleeve 3 is such that a width A from a central bent portion 10a to an end portion 10b of the dynamic pressure-generating grooves 8a is greater than a width B from the central bent portion 10a to an end portion 10c, and that the shapes of the grooves on either sides of the width A and the width B become unsymmetrical. In addition, the unsymmetrical grooves are formed as dynamic pressure-generating grooves that are located at the furthest position from the thrust bearing 1. With such construction, an oil moving direction, that is produced due to imbalance of dynamic pressure, is restricted to the thrust bearing side and a liquid level position of the lubricating oil is stabilized between 10a and 10b of the dynamic pressure-generating grooves 8a. 
Accordingly, in the dynamic pressure-type liquid bearing unit constructed as described above, by rotating a motor, a force is produced to move the lubricating oil toward the center of the bearing, and the height of the liquid level is shifted and the liquid level is stabilized.
However, at the same time as the motor is rotated, the liquid level of the lubricating oil shifts to a position wherein the dynamic pressures inside the dynamic pressure-generating grooves 8a and 8b balance each other, and therefore the dynamic pressure-generating grooves 8a and 8b are partially exposed to air so that the dynamic pressure-generating grooves 8a and 8b easily catch air and the air is trapped in the bearing.
If the trapped air remains in the bearing, there arises a problem that air pressure is generated to push the lubricating oil outside so that the lubricating oil flows out of opening portions.
It is an object of the present invention to solve the above-described problem and provide a dynamic pressure-type liquid bearing unit which can prevent lubricating oil from flowing outside by discharging air that is trapped in the bearing.
A dynamic pressure-type liquid bearing unit of the invention is characterized in that groove portions for removing air are formed on the outer surface of the stationary shaft.
According to the present invention, air that is trapped in the bearing can be discharged at all times and lubricating oil can be prevented from flowing out.
A dynamic pressure-type liquid bearing unit according to a first aspect of the present invention is constructed so that; a thrust plate is attached to one end side of a sleeve which is rotatably supported on a stationary shaft, and a thrust bearing attached to the stationary shaft is sandwiched by the sleeve and the thrust plate leaving a gap around the thrust bearing, unsymmetrical herringbone-shaped dynamic pressure-generating grooves are formed on at least one surface of the outer circumferential surface of the stationary shaft and the inner circumferential surface of the sleeve that is opposed to the outer circumferential surface, and lubricating oil is filled in a gap between the stationary shaft and the inner circumferential surface of the sleeve and the gap around the thrust bearing between the sleeve and the thrust plate, wherein a first capillary sealing portion, on which a tapered surface along the axial direction is formed, is formed between one end of the stationary shaft and the sleeve and a second capillary sealing portion, on which a tapered surface along the axial direction is formed, is formed between the other end of the stationary shaft and the thrust plate, and groove portions for removing air are formed on the outer surface of the stationary shaft between the first capillary sealing portion and the second capillary sealing portion.
According to this construction, even if air is trapped in the lubricating oil, the trapped air is discharged outside by the groove portions for removing air, therefore, expansion due to changes in temperature and air pressure can be suppressed and the lubricating oil is prevented from flowing outside.
A dynamic pressure-type liquid bearing unit according to a second aspect of the present invention is constructed so that; in the first aspect, the groove portions are formed on the side of the second capillary sealing portion.
A dynamic pressure-type liquid bearing unit according to a third aspect of the present invention is constructed so that; in the second aspect, the groove portions are formed to extend to the second capillary sealing portion.
According to this construction, air trapped in the lubricating oil can be discharged outside more easily.
A dynamic pressure-type liquid bearing unit according to a third aspect of the present invention is constructed so that; in any of the first through third aspects, the length of the second capillary sealing portion is made longer than the length of the first capillary sealing portion.
According to this construction, the retention volumes of the lubricating oil in the capillary sealing portions are increased, whereby the lubricating oil can be prevented from leaking outside.